Heat exchangers, boilers, combustion chambers, water heaters, and other similar devices control or alter thermal properties of one or more fluids. In some cases, one or two diffuser plates are disposed within these devices to hold one or more tubes (e.g., heat exchanger tubes, condenser tubes) in place. The diffuser plates help make the flow of fluids more uniform in the heat exchanger system. Diffuser plates can correct the flow direction of fluids inside the device. Diffuser plates can also help keep fluids from flowing through short cuts in the those devices.